Planetary type skew rolling mills are known in the art for reducing billets having solid cross-sections. Such mills have a hollow axis which permits the passing through of the material being milled. A roller carrier which is driven is mounted on the hollow axis. The roller carrier comprises three driven conical or skew rollers having a central rotational axis which is inclined relative to the hollow axis. These skew rollers are distributed around the hollow axis at 120.degree. spacings. These skew rollers are so inclined relative to the hollow axis that extensions of the roller axes intersect each other at equal short spacings between the intersection and the next adjacent end of the respective skew roller. Each of the skew rollers is arranged for cooperation with an adjustment device or screw down gear for the axial displacement of the skew roller shaft. Planetary gear wheels which revolve around the milling axis cooperate or mesh with a sun gear wheel.
In order to use such planetary skew rolling mills for rolling material having a full, solid cross-section as well as hollow blocks with thick walls for producing pipe stock, it is necessary to use skew rollers having special calibrations or a profiling or fairing which varies over the width of the skew roller, such width extending in the axial direction of the respective skew roller. During the operation of these skew rollers differing frictional coefficients occur over the width of the skew rollers. For example, at the intake zone of the rollers a certain traction is required, whereas in the transition area between the size reducing zone and the smoothing zone the skew roller surface shall be as smooth as possible. The smoothing surface of the skew roller itself shall also be as smooth as possible. Additionally, different types of skew rollers having different dimensions and surface configurations are required for different types of materials as well as for hollow blocks of the same material, but having different dimensions. Similarly, different types of skew rollers are required for manufacturing final products having different final cross-sections or dimensions.
As a result, it would be necessary to keep in stock a substantial number of different skew rollers having different dimensions and shapes. It seems that this large number was the reason for the fact that planetary type skew rolling mills have not been used heretofore for milling starting materials having a solid cross-sectional shape to manufacture stock having a predetermined final cross-section. Such planetary type skew rolling mills have also not been used for milling down hollow blocks to the size of tubular stock.